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The invention relates to actuation of a micro-electromechanical system (MEMS) device, and in particular to actuation of a two-axis tip-tilt MEMS mirror. The invention finds application to electrostatically actuated optical switching, but it is not so limited.
Schemes for electrostatic and magnetic actuation of two-axis tip-tilt MEMS mirrors using four actuation elements are well known. A four element configuration has the advantage of straight-forward symmetry with respect to orthogonal tip-tilt axes, so that the transformation function between tilt orientation and applied voltage or current at each actuation element is relatively straight-forward. The general method of actuation using four electrodes is to actuate electrodes in pairs on a common side of an axis to tilt about the axis.
In order to control the individual actuation elements, each element requires its own voltage or current supply-line and associated drive circuitry. In the case of an optical switch using arrays of closely spaced mirrors, the large number of lines and drivers becomes a limiting factor in system design. Due to packing constraints, routing of the lines becomes challenging as the number of MEMS devices in the array is increased. Furthermore, the system cost scales as the number of drivers is increased due the larger number of electronic components. What is needed is a scheme to reduce the number of drivers and lines to reduce both the interconnect and driver problems.
According to the invention, a method and apparatus are provided for driving a two-axis MEMS mirror using three non-contact actuation elements or electrodes. A differential bi-directional mirror control uses three actuation drive signals biased at a suitable value. Transformation functions map two-axis tip-tilt commands to three actuation drive signals for selected electrode orientations and sizes.
Although the theoretical basis as presented here is for using three electrodes in an electrostatic drive, the theory is applicable to other cases involving two-axis tip-tilt devices, including electromagnetic actuators for MEMS devices. However, active bi-direction motion to attain full displacement can be achieved with electrostatic electrodes (which only produce attractive force) only if, for each axis of angular rotation, the electrode centers of two electrodes are on opposites sides of the axis. Therefore, the disclosure is to be understood to address the case of using three electrodes to drive a two-axis tip-tilt device of this type of axis and electrode configuration.
The invention will be better understood by reference to the following detailed description in connection with the accompanying embodiments.